In welding of high tensile steel, preheating/interpass temperature must be strictly managed from the viewpoint of preventing low-temperature cracking of a weld metal portion, which is a cause of low operation efficiency. Recently, steel to be used for a welded structure has increasingly higher strength, and a weld metal is accordingly required to have higher strength (for example, HT780: 780 MPa class high strength steel).
Such increased strength tends to lower resistance to low-temperature cracking. The resistance to low-temperature cracking is therefore necessary to be improved. In particular, since gas shielded arc welding using a flux cored wire is excellent in welding workability, a technique for securing the resistance to low-temperature cracking is required for a weld metal formed by the gas shielded arc welding.
Such low-temperature cracking is estimated to be caused by diffusible hydrogen that is segregated in grain boundaries and lowers grain boundary strength, (hereinafter, this is referred to as “hydrogen embrittlement”). How to decrease the diffusible hydrogen is therefore an important point to improve the resistance to low-temperature cracking.
Consequently, susceptibility to hydrogen embrittlement of weld metal must be lowered in order to improve the resistance to low-temperature cracking of the weld metal. Various techniques have been therefore proposed.
For example, PTL 1 discloses a technique that prevents low-temperature cracking by dispersing Mo carbide particles (carbide particles containing Mo) having high hydrogen trap ability in a weld metal. However, this technique must adopt a special welding method in order to disperse the Mo carbide particles. That is, steel pieces must be butted together and then jointed from the inside by submerged arc welding. Hence, the technique is not applicable for typical welding of steel.
PTL 2 proposes a technique that prevents the low-temperature cracking by controlling cooling time during welding operation. This technique requires strict operation control depending on components, and thus has a problem of a high work load.
PTL 3 proposes a technique that prevents the low-temperature cracking by adjusting a fraction of retained austenite, which traps diffusible hydrogen, to 1% or more in a weld metal. However, this technique is based on double one layer seam welding of a steel pipe, and is therefore inapplicable for typical welding of steel.
PTL 4 proposes a technique that prevents the low-temperature cracking by decreasing the amount of diffusible hydrogen and appropriately controlling strength and a chemical composition. However, this technique is also limitedly applicable for actual operation sites since a strength level to be satisfied varies depending on components.
Each of the previously proposed techniques described above aims to improve the resistance to low-temperature cracking. In actual welding operation, however, the amount of hydrogen in a weld metal may increase due to various factors. More essentially, therefore, the resistance to hydrogen embrittlement must be improved.
Furthermore, HT780 class steel is recently expansively applied to a weld metal used in an offshore structure. Such a weld metal is required to have excellent resistance to hydrogen embrittlement at the strength of 780 MPa class. Furthermore, the weld metal is preferably required to have excellent low-temperature toughness so as to be durable in a cold district.